Rename ModelElement field element2 -> element (#4000)

Also had to rename some local variables for this refactor, to avoid
shadowing.

Author: keertip

lib/src/element_type.dart

@@ -322,7 +322,7 @@ abstract class DefinedElementType extends ElementType {
   @internal
   @override
   CommentReferable get definingCommentReferable =>
-      ModelElement.forElement(modelElement.element2, packageGraph);
+      ModelElement.forElement(modelElement.element, packageGraph);
 }
 
 /// Any callable [ElementType] will mix-in this class, whether anonymous or not,

lib/src/generator/generator_backend.dart

@@ -85,9 +85,9 @@ abstract class GeneratorBackend {
         isSidebar ? '' : data.htmlBase,
       );
     }
-    var element = data.self;
+    var e = data.self;
     writer.write(filename, content,
-        element: element is Warnable ? element : null);
+        element: e is Warnable ? e : null);
   }
 
   /// Emits JSON describing the [categories] defined by the package.

lib/src/generator/generator_frontend.dart

@@ -149,7 +149,7 @@ class GeneratorFrontEnd implements Generator {
       for (var lib in package.libraries.whereDocumented) {
         if (!multiplePackages) {
           logInfo('Generating docs for library ${lib.breadcrumbName} from '
-              '${lib.element2.firstFragment.source.uri}...');
+              '${lib.element.firstFragment.source.uri}...');
         }
         if (!lib.isAnonymous && !lib.hasDocumentation) {
           packageGraph.warnOnElement(lib, PackageWarning.noLibraryLevelDocs);